System and method for catching top hosts

ABSTRACT

Disclosed are a system and a method for catching top users. The system may comprise: a filter ( 10 ) configured to sample flows from the hosts and remove the flows not satisfying a constraint; an tracker ( 20 ) configured to record a first estimated flow count for each host and to determine a first set of hosts from the plurality of hosts in term of the estimated flow count; and an estimator ( 30 ) configured to determine a second estimated flow count for each of the determined hosts and select a second set of hosts from the determined hosts based on the second estimated flow count.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional Application No. 61/150,698 filed on Feb. 6, 2009, of which the contents are incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present application relates to a system and a method for catching top hosts in the network.

BACKGROUND

Efficiently and accurately identifying hosts that are spreading the largest amount of flows during an interval of time is very important for managing a network and studying host behaviors on application level, ranging from detecting DDoS attack, worm propagation, peer-to-peer hot spots and flash crowds. No previous work has been able to efficiently and accurately identify the top spreaders at very high link speed, for example, 10 to 40 Gbps.

Considering the case of finding hosts who are spreading a large amount of flows, FIG. 1 shows a scenario: hosts in a local ISP network communicate with other hosts in the global Internet through a high speed link. As shown, host₃ is communicating with a lot of hosts, and may be a very popular web server, or may be initiating or under DDoS attack. It is required to quickly and efficiently identify such kind of hosts in a network, and know how severe the situation is.

There has been a lot of works on measurement of traffic statistics for network management, security, and its evolvement. The size distribution and matrices of flows from the hosts may help a network to provide and engineer traffic thereof. Finding flows that have a large number of packets is useful in billing and accounting. It has also been shown that flow level communication patterns may further reveal application level behaviors of each host.

Typically, flows of small sizes are more interesting to security related problems. For example, a host scanning a port or address typically sends only a very small number of packets to each victim, to keep the overhead small and lower the chance to be detected. A SYN flood DDoS attack typically contains only one SYN packet in each attack flow, and the acknowledged ACK packets are ignored. A newly exposed TCP attack uses many low rate TCP sessions to exhaust resources of the victim, and during a small interval, these TCP flows can also be viewed as small flows. P2P applications tend to contact some servers or other peers to exchange control information in a periodical fashion, and such control messages typically contain a small number of packets.

Currently, there are some problems of detecting super hosts with small flows in the field.

SUMMARY

In one aspect, a system for catching top hosts from a plurality of hosts may comprise:

a filter configured to sample flows from the hosts and remove the flows that not satisfying a first constraint;

a tracker configured with a data structure for recording a first estimated flow count for each host and configured to determine a first set of hosts from the plurality of hosts in term of the estimated flow count; and

an estimator configured to determine a second estimated flow count for each of the determined hosts and select a second set of hosts from the determined hosts based on the second estimated flow count.

In the other aspect, a method for catching top hosts from a plurality of hosts may comprise:

sampling a plurality of packets from the hosts during a determined interval of time;

determining a difference between a count and a count error for each of the hosts based on the sampled packets;

ranking the hosts based on the determined difference to identify a first set of hosts in the ranked hosts;

estimating a flow count for each of the first set of hosts; and

selecting a second set of hosts from the first set of hosts as the top hosts based on the estimating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a communication scenario for hosts in the prior art;

FIG. 2 is a block diagram schematically illustrating the system architecture according to one embodiment of the application;

FIG. 3 is a block diagram schematically illustrating the filter of FIG. 2 according to one embodiment of the application;

FIG. 4 is a block diagram schematically illustrating the tracker of FIG. 2 according to one embodiment of the application;

FIG. 5 schematically illustrates a data structure of system of FIG. 2 according to one embodiment of the application;

FIG. 6 is a diagram illustrating a typical scenario for the using of the filter of FIG. 2;

FIG. 7 is a block diagram schematically illustrating the estimator of FIG. 2 according to one embodiment of the application;

FIG. 8 is a block diagram schematically illustrating the flowchart of updating the estimator of FIG. 2 according to one embodiment of the application;

FIG. 9 is a block diagram schematically illustrating the flowchart of the singleton flow count estimation algorithm according to one embodiment of the application.

FIG. 10 is another diagram illustrating a typical scenario for the using of the filter of FIG. 2;

FIG. 11 illustrates a mapping of the counters in the filter according to one embodiment of the application; and

FIG. 12 illustrates a permutation of the counters in the estimator as shown in FIG. 5.

DETAILED DESCRIPTION

Hereinafter, a detailed description of implementations will be given with reference to the appended drawings and embodiments.

For the purpose of description, the following definitions are given first.

A scanner is defined as a host or a port on a host, acting as a “user” that is communicating with other hosts or ports. In the application, there is almost no difference when handling the two situations, except whether the user ID comes from the IP address or the IP address together with the port number. Hereinafter, the former case is used to discuss the embodiment of the application.

A flow is defined as a combination of fields in a packet, for example, IP address, port, TCP flags, or even packet content. Different applications may have interests in different fields, and the unique combination of these fields forms a flow ID. In the application, the flows of size i are considered, where i is a small integer. Such flows are referred as small flows. For ease of discussion, the flows of size 1, as referred as singleton flows, are used to describe the embodiments of the application.

A scanner or user is super in terms of the number of its singleton flows. A scanner or user is top if and only if its singleton flow count is above a threshold. Although finding all users with singleton flow counts above a threshold can be useful, in practice, it is hard to find an appropriate threshold to classify the users. For example, given a threshold, there are often either too many or too few users. Only considering ranking based top users may suffer that, from another point, no meaningful results exist at all if all users only have a small number of singleton flows. So, in the application, these two conditions are combined together, and the exact value of a threshold is less important.

FIG. 2 schematically illustrates a system 1000 for determining top hosts according to an embodiment of the application. As shown, the system 1000 comprises a filter 10, a tracker 20 and an estimator 30. The filter 10 is configured to sample flows from a plurality of hosts and remove the flows not satisfying a first constraint during a determined interval of time, which will be discussed hereinafter. The tracker 20 is configured with a data structure for recording a first estimated flow count for each of the hosts and configured to determine a first set of hosts (for example, the top-2k hosts) from the plurality of hosts in term of the estimated flow count. The estimator 30 is configured to determine a second estimated flow count for each of the determined hosts and select a second set of hosts (for example, the top-k hosts) from the determined hosts based on the determined second flow counts. k is a small number, e.g. 10 or 20, which is most useful for network management and security.

1. Filter 10

As shown in FIG. 3, the filter 10 is configured with a sampling unit 101, a determining unit 102 and an accounting unit 103. The sampling unit 101 operates to sample the arrival packets and to calculate a value of V₁ based on the flow ID f of the packets by the following equation:

V _(f) =h(f)   1)

Where h( ) is a uniform hash function that may calculate, for example, a hash result of 32 bits.

A plurality of counters are arranged in the accounting unit 103 and are represented mathematically as an array c[1 . . . m], where each counter has two bits. The determining unit 102 operates to determine whether the calculated V_(f) is less than a predetermined sampling threshold T₁. If yes, this packet is sampled and then the accounting unit 103 checks whether the array c[1 . . . m] should be updated, which will be discussed in detailed as below. If no, this packet is discarded. If the intended sampling rate is r and the output range of h( ) is from 1 to H, then T₁ should be set to rH, such that one out of 1/r flows is sampled. The sampling rate r should be inversely proportional to the link speed. For example, on high speed links such as OC768, r can be set to 1/16, while on links with speed of or less than OC48, r can be set to 1.

At beginning of the sampling, all counters of c[1 . . . m] are set to be zero, so that each counter in array c[1 . . . m] may record up to three. Packets of the same flow f are mapped to the same position l=h_(m)(f), where h_(m)( ) can calculate a hash result in the range from 1 to m, and can be a prefix of h( ) in implementation. The element e[l] is used to record the number of packets already mapped to this position. In particular, we use c_(t) to represent the counters in the filter 10 when seeing the t-th sampled packet and assuming the packet is from host s. If c_(t)[l]≦1, then the accounting unit 103 sets c_(t+1)[l]=c_(t)[l]+1.

When considering small flows of no more than q packets, the accounting unit 103 sets c_(t+1)[l]=c_(t)[l]+1 when c_(t)[l]≦q, and each counter needs at least ┌log₂(q+2)┐ bits where ┌x┐ means the smallest integer no smaller than x.

2. Tracker 20

Referring to FIG. 4, the tracker 20 maintains a hash table 201 and a min-heap 202. The key of a node in the hash table 201 is host ID (i.e. parameter s), and the value of a node in the hash table 201 is the corresponding position of the node in the min-heap 202. As shown in FIG. 5, a node in the min-heap 202 has three fields, its host ID, its count (not necessary to be an integer), and its possible error of count err. The count and err of s in the min-heap 202 act as a coarse estimation for the flow count of s. In the min-heap 202, at most z nodes are kept for z different hosts. The count's and err's in the min-heap 202 may also be represented as two arrays count[1 . . . z and err[1 . . . z] for efficient implementation.

The counter number z should be set to

${z = {\min \left( {{A},\left\lceil \frac{n}{T} \right\rceil} \right)}},$

where |A| is the number of hosts and n is the total number of flows, and T is the threshold such that hosts with more than T singleton flows are considered to be “super”. For each update, the worst case processing time is O(log ≈). Since z is very small, the hash table 201 can be stored in a TCAM with O(1) performance. For strict time requirement, sampling in the filtering 10 may reduce processing overhead.

As shown in FIG. 4, the tracker 20 is also provided with a detecting unit 203 for detecting the change of the counter values in the accounting unit 103. Specifically, when c[l] (where l=h_(m)(f)) in the accounting unit 103 is changed, the field of count in the min-heap 202 may be updated accordingly. In particular, for each update for host s triggered by the filter 10, ifs is not kept in the ID field of the table 201 and the min-heap 202, the ID with the minimal count (let min be this value) in the min-heap 202 and table 201 is replaced by s, the corresponding count in the min-heap 202 is updated as below, and the corresponding err is set as err_(s)=min. However, if s is already kept in the ID field of the table 201 and the min-heap 202, only the count of s (that is count_(s)) is updated as below.

When the t-th packet with host ID s and flow ID f arrives, and c_(t)[h_(m)(f)] in the accounting unit 103 is changed,

$\begin{matrix} {{{{{If}\mspace{14mu} {c_{t}\left\lbrack {h_{m}(f)} \right\rbrack}} = 0},{{count}_{s} = {{count}_{s} + {\frac{m}{y_{0}(t)} \times \left( {\frac{y_{1}(t)}{y_{0}(t)} + 1} \right)}}}}{{{{If}\mspace{14mu} {c_{t}\left\lbrack {h_{m}(f)} \right\rbrack}} = 1},{{count}_{s} = {{count}_{s} - \frac{m}{y_{0}(t)}}}}} & \left. 2 \right) \end{matrix}$

where m represents the number of counters in the filter 10, and y₀(t) and y₁(t) are the number of counters in c_(t) with value 0 and 1 respectively. Herein the value of counts is the flow count of host s.

Taking FIG. 6 as an example, for the 2-bits counter values, since only 0 and 1 are useful for estimating singleton flow count, the application uses 2 to mean any value large than 1, while 3 is not used. Gray squares represent packets that have arrived, and squares with characters represent forthcoming packets. Different gray levels or characters represent different flows. Although each is the first packet of a flow, A₁ and E₁ see a value of 0, but B₁ and D₁ see a value of 1, and C₁ sees a value of 2. As the second packet of a flow, B₂ will see a value of 2, but E₂ will see a value of 1.

When considering small flows that have no more than q packets, equation 2) should be changed accordingly to:

If c _(t) [h _(m)(f)]=i and i≦q, count_(s)=count_(s) +U _(i,q)(t)   3)

where U_(i,q)(t) is determined by i, q, y₀(t), . . . , u_(q)(t), m. The counter in the filter 10 should be at least ┌log₂(q+2)┐ bits where ┌r┐ means the smallest integer no smaller than x. The exact formulas for computing U_(i,q)(t) are provided as equation 8) in the mathematical proof.

As shown in FIG. 4, the tracker 20 also comprises a selecting unit 204. The selecting unit 204 is configured to rank all of the hosts based on the estimated flow counts, namely count_(s)'s, in a descending or ascending order. The top-2k hosts in the ranked hosts are identified by the selecting unit 204 as potential top spreaders, where k is an integer. Although theoretically, k can be selected from 1 to the total number of hosts, here it only considers the situation where k is a small constant number, e.g. 10 or 20, when the network management and security is concerned.

The detailed architecture with data structures for the filter 10 and tracker 20 are shown in FIG. 5, wherein some units therein are omitted for the purpose of clarity.

3. Estimator 30

The estimator 30 is used to select, for example, top-k hosts from, for example, the top-2k potential top spreader identified by the tracker 20. As shown in FIG. 7, the estimator 30 includes a counter 301 configured with w×d counting units (d columns, each column has w counting units), with each counting unit represented by 2 bits. All of the counting units are initialized to zero. Besides the counter 301, the estimator 30 also includes a detecting unit 302, a selecting unit 303 and an updating unit 304.

The detecting unit 302, the selecting unit 303 and the updating unit 304 cooperate to update the counting units in the first counter 301. FIG. 8 illustrates the updating of the first counter 301 according to one embodiment of the application. In step S801, when a packet from host s arrives (notice that different from the filter 10, packets are used with sampling), the detecting unit 302 operates to detect whether s is tracked in the tracker 20 and the coarse flow count of s, that is count_(s)−err_(s) in the min-heap 202 of the tracker 20, is above an estimating threshold T₂. When the detecting unit 302 detects count_(s)−err_(s) in the min-heap 202 of the tracker 20 is above T₂, in step S802, it uses for example, 3 hash functions h₁(s), h₂(s), h₃(s) to select 3 columns of counting units from d columns by a rule of

columns_(s) =h _(i)(s) for i=1 to 3   4)

where columns_(s) represents the selected columns of the counting units where an input parameter is the host ID s. In the embodiment, for the purpose of illustration, the three columns are denoted as M_(h) ₁ _((s)), M_(h) ₂ _((s)) and M_(h) ₃ _((s)).

In step S803, the selecting unit 303 operates to select a row of counting units from the selected three columns (M_(h) ₁ _((s)). M_(h) ₂ _((s)). M_(h3(s))) in the same level. In particular, f=h_(w)(f) is used to represent the location of row for the selected counting units, where h_(w)( ) can calculate a hash result in the range from 1 to w, and can be a prefix of h( ) in implementation. Now the three counting units M_(h) ₁ _((s))[l], M_(h) ₂ _((s))[l], M_(h) ₃ _((s))[l] are selected.

In step S804, for a new packet from s with flow ID f, the updating unit 304 operates to set the three selected counting units as below.

For i=1 to 3

If M _(h) _(i) _((s)) 8 l]<=1, M _(h) _(i) _((s)) [l]=M _(h) _(i) _((s)) [l]+b 1   5)

When considering small flows of no more than q packets, equation 5) should be changed accordingly to

If M _(h) _(i) _((s)) [l]<=q, M _(h) _(i) _((s)) [l]=M _(h) _(i) _((s)) [l]+1   6)

Each counting unit in the estimator 30 should be at least ┌log₂(q+2)┐ bits where ┌x┐ means the smallest integer no smaller than x.

Referring to FIG. 7 again, the estimator 30 may further comprise an estimating unit 305. The estimating unit 305 is configured to estimate flow counts of the selected 2k hosts.

FIG. 9 illustrates the estimating processing for the estimating unit 305. In this exemplified example, consider the three columns (M_(h) _(i) _((s)), M_(h) ₂ _((s), M) _(h) ₃ _((s))) that host s is mapped to and denote them by M₁, M₂ and M₃ (S901). Then, in step S902, M₁₂, M₁₃, M₂₃ and M₁₂₃ are calculated, where M₁₂[l]=M₁[l]+M₂[l], M₁₃[l]=M₁[l]+M₃[l], M₂₃[l]=M₂[l]+M₃[l], and M₁₂₃[l]=M₁[l]+M₂[l]+M₃[l], for 1≦l≦w. In step S903, determine whether any one of M₁₂, M₁₃, M₂₃ and M₁₂₃ overflows (it overflows if there is no counting unit with value 0). If yes, we can not use them for estimating, and then in step S904, use (count_(s)−err_(s))/r from the tracker 20 as the flow count of host s. Otherwise, in step S905, a temporarily estimated flow count St is calculated by a rule of

St=(|M ₁ |+|M ₂ |+|M ₃ |−|M ₁₂ |−|M ₁₃ |−|M ₂₃ |+|M ₁₂₃|)/3,   7)

where |M_(A)|=w×(#counters in M_(A) with value 1)/(#counters in M_(A) with value 0) and r is the sampling rate for the sampling unit 101.

In step S906, T₂/r is added to St, and the result is used as an estimated flow count for the host s.

When considering small flows with no more than q packets, equation 7) should be changed accordingly. The exact formulas for computing |M_(A)| and St are provided as equations 9) ˜15) in the mathematical proof.

Referring to FIG. 7 again, the estimator 30 may further comprise an outputting unit 306. The estimating unit 306 is configured to sort the host IDs in a descending or ascending order based on the estimated flow counts for the hosts and selecting top-k hosts from the top-2k hosts.

As mentioned in the above, the counter number z in the tracker 20 should be set to

${z = {\min \left( {{A},\left\lceil \frac{n}{T} \right\rceil} \right)}},$

where |A| is the number of hosts and n is the total number of flows, By neglecting time for hash computations and update on y₀ and y₁, it can be seen that for each sampled flow (1 out of 1/r), where r is the rate of sampling, at most two packets can cause update on the filter 10 and the tracker 20, where each update needs at most one read and one write on the filter 10, and 1+log(z) r/w on the tracker 20. And for a small number of hosts, each packet of them needs one read on the tracker 20 and 3 r/w on the estimator 30. The total memory is 2m+240z+2dw bits (for small flows with no more than q packets, the memory required is ┌log₂(q+2)┐m+240z+┌log₂(q+2)┐dw bits).

For a practical situation such as n=2²⁰ and |A|=20K, we set r=1/16, m=2²², c=2000, w=4096 and d=1024, the memory needed is 1148 KB, which can be affordable in SRAM, and the update speed can keep up with 40 Gbps. The identified top-20 scanners are almost always correct, and the flow count estimation at an average relative error of 3%. Using more memory can lead to better performance.

More importantly, the system and the algorithms used therein can perform on very high speed links, with a large number of hosts and flows, e.g. 10 Gbps=40 Gbps, 1M flows and 200K hosts, which is the real situation happening on ISP backbones.

In case that a host has a very large number of flows (e.g. more than wln(w)), then the estimator 30 may be filled up, and we just use the counter provided by the tracker 20.

Hereinabove, the systems 1000 and the cooperation for the components in the systems have been discussed. Now, some mathematical supports for some of the above equations are given as below.

1. Mathematical Support for Equation 2) and 3)

We consider an array of in counters, and use y_(k)(t) and c_(j)(t) to explicitly denote the number of counters with value k and the value of the j-th counter right before the t-th packet arrival, respectively. As shown in FIG. 10, B₁, the t-th packet, sees a counter value of two. Gray squares represent packets that have arrived before B₁, squares with characters represent forthcoming packets, and different patterns or characters represent different flows. For clarity we have omitted the uniform hash function that maps flows to counters.

First we note that, the counter value that any packet sees is independent of the arriving order of its preceding packets. We denote the set of flows by

, and denote the x-th packet of flow f by f_(x). We also denote the t-th packet among all packets by e_(t) which sees value v_(t), so the event “┌f ∈

, e_(t)=f_(x)” (abbreviated to “e_(t)=f_(x)”) means “the t-th packet is the x-th packet of some flow f, and v_(t)=c_(hm(f))(t). Now we have some theorems about the probability of this event.

Theorem 1. The conditional probability that the t-th packet e_(t) sees a counter value of z, given that e_(t) is the x-th packet of some flow, is:

${\Pr \left( {v_{t} = {{ze_{t}} = f_{x}}} \right)} = \left\{ \begin{matrix} {0,} & {{{{if}\mspace{14mu} z} < {x - 1}};} \\ {y/m} & {{{{{if}\mspace{14mu} z} \geq {x - {1\mspace{14mu} {and}\mspace{14mu} x}}} = 1};} \\ {p,} & {{{{if}\mspace{14mu} z} \geq {x - {1\mspace{14mu} {and}\mspace{14mu} x}} > 1};} \end{matrix} \right.$

where y=y_(z−x+1)(t) is the number of counters with value z−x+1 right before this packet arrives, (y−1)/m≦p≦(y+1)/m and with high probability, p=y/m. Particularly, for x=1, that is for the first packet of each flow, we have Pr(v_(t)=z|e_(t)=f₁)=y_(z)(t)/m.

Proof of Theorem 1:

(1) Since the preceding x−1 packets belonging to the same flow as e, map into the same position, z is at least x−1, so the probability is always zero when z<x−1.

(2) For x=1, which means e_(t) is the first packet of some flow, since the position mapping is uniform and is independent of any proceeding packets, the probability that it sees value z is just the percentage of counters with value z at that time, that is, y_(z)(t)=m.

(3) For x>1, however, the mapping of B_(x) is dependent on B₁ and the probability is not simply y_(z)(t)/m. Let us focus on the counter which B₁, B₂, . . . are mapped to, as shown in FIG. 11. Due to collisions, there may be some other flows (gray squares) mapped to that counter.

In the first row, the t-th packet B_(x) is the x-th packet of flow B, and it sees a counter value of z′. We can always reorder its preceding packets without changing the counter values. So we can look for a particular ordering to help the calculation of the probability of z′=z. Specifically, we put all the preceding x−1 packets of the same flow right before this packet, as shown in the second row of FIG. 11. Then B₁, the first packet of flow B, would have seen a counter value of z′−x+1, as shown in the third row. For this particular reordering, we have Pr(v_(t)=z|e_(t)=f^(x))=Pr(c_(h(f))(t−x+1)=z−x+1|e_(t)=f_(x)), that is, the probability that B_(x) sees value z equals the probability that B₁ sees value z−x+l. It should be noted that all the other counters remain unchanged after this reordering. So if in the first and second row there are y=y_(z−x+1)(t) counters having value z−x+l right before B_(x) (the t-th packet) arrives, then in the third row there must be at lease y−1 and at most y+1 counters having a value of z−x+1 right before B₁ arrives, since no other counter except the one B₁ (or B_(x)) is mapped to may change its value. That is to say, (y−1)/m≦Pr(c_(j)(t−x+1)=z−x1|e_(t)=f_(x))≦(y+1)/m, and hence (y−1)/m≦p≦(y+1)/m. And with high probability, p=y/m.▪

Then we can rewrite the probability that the t-th packet is the q-th packet of some flow:

Theorem 2. Pr (c_(t)=f_(q)) can be decomposed as Pr(e_(t)=f_(q))≈Σ_(i=0) ¹⁻¹Pr(v_(t)=i)×Y_(i,q)(t), where Y_(i,q)(t) is a fixed polynomial expression of m/y₀(t), (t)/y₀(t), y₂(t)/y₀(t), . . . , y_(q−i−1)(t)/y0(t).

Proof of Theorem 2: We prove by induction.

(1) For q=1, since Pr(v_(t)=0. e_(t)=f₁)=Pr(v_(t)=0), we have

${{\Pr \left( {e_{t} = f_{1}} \right)} = {\frac{\Pr \left( {{v_{t} = 0},{e_{t} = f_{1}}} \right)}{\Pr \left( {v_{t} = {{0e_{t}} = f_{1}}} \right)} = {{\Pr \left( {v_{t} = 0} \right)} \times \frac{m}{y_{0}(t)}}}},$

so the theorem holds for q=1.

(2) For q>1, we have,

$\begin{matrix} {{\Pr \left( {e_{t} = f_{q}} \right)} = \frac{\Pr \left( {{v_{t} = {q - 1}},{e_{t} = f_{q}}} \right)}{\Pr \left( {v_{t} = {{{q - 1}e_{t}} = f_{q}}} \right)}} \\ {\approx \frac{{\Pr \left( {v_{t} = {q - 1}} \right)} - {\sum\limits_{x = 1}^{q - 1}{\Pr \left( {{v_{t} = {q - 1}},{e_{t} = f_{x}}} \right)}}}{{y_{0}(t)}/m}} \\ {= \frac{{\Pr \left( {v_{t} = {q - 1}} \right)} - {\sum\limits_{x = 1}^{q - 1}{{\Pr \left( {v_{t} = {{{q - 1}c_{t}} = f_{x}}} \right)} \times {\Pr \left( {c_{t} = f_{x}} \right)}}}}{{y_{0}(t)}/m}} \\ {{\approx {{{\Pr \left( {v_{t} = {q - 1}} \right)} \times \frac{m}{y_{0}(t)}} - {\sum\limits_{x = 1}^{q - 1}{{\Pr \left( {c_{t} = \int_{x}} \right)} \times \frac{y_{q - x}(t)}{y_{0}(t)}}}}},} \end{matrix}$

where we approximate Pr(v_(t)=z↑e_(t)=f_(x)) by y_(z−x+1)(t)/m. By induction, our theorem holds for any q.▪

By applying an expectation to theorem 2, we have

Corollary 1. E(I(e_(t)=f_(q)))≈Σ_(i=0) ^(q−1)E(I(v_(t)=i)×Y_(i,q)(t).

The exact expressions of Y_(i,q)(t)'s are fixed and can be pre-computed. Particularly, for q=1 and 2, we have

${{E\left( {I\left( {e_{t} = f_{1}} \right)} \right)} = {{E\left( {I\left( {v_{t} = 0} \right)} \right)} \times {m/{y_{0}(t)}}}},{{e\left( {I\left( {e_{t} = f_{2}} \right)} \right)} \approx {\frac{E\left( {I\left( {v_{t} = 1} \right)} \right)}{{y_{0}(t)}/m} - {\frac{E\left( {I\left( {v_{t} = 0} \right)} \right)}{{y_{0}(t)}/m} \times {\frac{y_{1}(t)}{y_{0}(t)}.}}}}$

Now let us use this information to count specific flows. The number of flows with only one packet is:

$\begin{matrix} {n_{1} = {E\left( n_{1} \right)}} \\ {= {E\left( {{\sum\limits_{t}{I\left( {e_{t} = f_{1}} \right)}} - {\sum\limits_{t}{I\left( {e_{t} = f_{2}} \right)}}} \right)}} \\ {= {{\sum\limits_{t}{E\left( {I\left( {e_{t} = f_{1}} \right)} \right)}} - {\sum\limits_{t}{E\left( {I\left( {e_{t} = f_{2}} \right)} \right)}}}} \\ {\approx {{\sum\limits_{t}\left( {\left( {\frac{y_{1}(t)}{y_{0}(t)} + 1} \right) \times \frac{E\left( {I\left( {v_{t} = 0} \right)} \right)}{{y_{0}(t)}/m}} \right)} -}} \\ {{\sum\limits_{t}\frac{E\left( {I\left( {v_{t} = 1} \right)} \right)}{y_{0}\; {(t)/m}}}} \\ {{= {E\left\lbrack {{\sum\limits_{{t:v_{t}} = 0}\left( {\left( {\frac{y_{1}(t)}{y_{0}(t)} + 1} \right) \times \frac{m}{y_{0}(t)}} \right)} - {\sum\limits_{{t:v_{t}} = 1}\frac{m}{y_{0}(t)}}} \right\rbrack}},} \end{matrix}$

Now we have decomposed n₁ into the expression of y₀(t), y₁(t) and y₁(t), which are all observable: the number of counters with value zero and one, and the value that packet e_(t) sees. Due to the unbiasedness of expectation, we have

Conclusion 1. If the t-th packet sees a counter value of zero, add

$\left( {\frac{y_{1}(t)}{y_{0}(t)} + 1} \right) \times \frac{m}{y_{0}(t)}$

to the singleton flow count. If it sees a counter value of one, deduct

$\frac{m}{y_{0}(t)}$

form the singleton flow count. This procedure gives us an asymptotically unbiased estimator for singleton flow count,

${\hat{n}}_{1} = {{\sum\limits_{{t:v_{t}} = 0}\left( {\left( {\frac{y_{1}(t)}{y_{0}(t)} + 1} \right) \times \frac{m}{y_{0}(t)}} \right)} - {\sum\limits_{{t:v_{t}} = 1}{\frac{m}{y_{0}(t)}.}}}$

And this is exactly the formulas we used in equation 2).

Now for flows with at most q packets, we have

$\begin{matrix} {{\sum\limits_{k = 1}^{q}n_{k}} = {E\left( {\sum\limits_{k = 1}^{q}n_{k}} \right)}} \\ {= {E\left( {{\sum\limits_{t}{I\left( {e_{t} = f_{1}} \right)}} - {\sum\limits_{t}{I\left( {e_{t} = f_{q + 1}}\; \right)}}} \right)}} \\ {= {{\sum\limits_{t}{E\left( {I\left( {e_{t} = f_{1}} \right)} \right)}} - {\sum\limits_{t}{{E\left( {I\left( {e_{t} = f_{q + 1}} \right)} \right)}.}}}} \end{matrix}$

Using corollary 1, we further get

$\begin{matrix} {{\sum\limits_{k = 1}^{q}n_{k}} = {\sum\limits_{t}{\sum\limits_{i = 0}^{q}\left( {{E\left( {I\left( {v_{t} = i} \right)} \right)} \times {U_{i,q}(t)}} \right)}}} \\ {{= {E\left( {\sum\limits_{i = 0}^{q}{\sum\limits_{{t:v_{t}} = i}{U_{{i,q}\;}(t)}}} \right)}},} \end{matrix}$

where U_(i,q)(t) is also a fixed polynomial expression of m/y₀(t), y₁(t)/y₀(t), y₂(t)/y₀(t), y_(q−i−1)(t)/y₀(t). Again, we can draw a conclusion on counting small flows:

Conclusion 2. Asymptotically, n_(1˜q), the number of small flows that having no more than q packets, can be unbiasedly estimated by summing up U_(i,q)(t)'s for those packets that see a counter value of i(i≦q). That is, {circumflex over (n)}_(1˜q)=Σ_(i=0) ^(q)Σ_(t:v) _(t) _(=i)U_(i,q)(t). And this is the formular we used in equation 3).

We note here that, the variance is also a polynomial of m/y₀(t) and y_(i)(t)/y₀(t)'s.

Particularly, when counting singleton flows, the variance is

${\sum\limits_{{t:e_{t}} = f_{1}}\left\lbrack {{\left( {\left( {\frac{y_{1}(t)}{y_{0}(t)} + 1} \right)^{2} + \frac{y_{1}(t)}{y_{0}(t)}} \right) \times \frac{m}{y_{0}(t)}} - 1} \right\rbrack} + {\sum\limits_{{t:e_{t}} = f_{2}}{\left( {\frac{m}{y_{0}(t)} - 1} \right).}}$

So if at least half of the counters are empty, that is y₀(t)≧m/2≧y_(i)(t), ∀i≧1, we can derive a loose (but already good enough) bound on the variance. Only ┌log(q+2)┐ bits are needed for each counter, since we only need to record up to q+1.

The formulas of Y_(i,q)(t) and U_(i,q)(t) can be deducted from Theorem 2, and are:

$\begin{matrix} {{{Y_{{q - 1},q} = \frac{m}{y_{0}}},{Y_{{q - 2},q} = {{- 1} \times \left\lbrack {Y_{{q - 1},q} \times \frac{y_{1}}{y_{0}}} \right\rbrack}},{Y_{{q - 3},q} = {{- 1} \times \left\lbrack {{Y_{{q - 2},q} \times \frac{y_{1}}{y_{0}}} + {Y_{{q - 1},q} \times \frac{y_{2}}{y_{0\;}}}} \right\rbrack}},\ldots}{{Y_{{q - i},q} = {{- 1} \times \left\lbrack {\sum\limits_{j = 1}^{i - 1}{Y_{{q - {({i - j})}},q} \times \frac{y_{j}}{y_{0\;}}}} \right\rbrack}},\ldots}{{Y_{0,q} = {{{- 1} \times {\left\lbrack {\sum\limits_{j = 1}^{q - 1}{Y_{j,q} \times \frac{y_{j}}{y_{0}}}} \right\rbrack.U_{0,q}}} = {{m/y_{0}} - Y_{0,{q + 1}}}}},{U_{i,q} = {- Y_{i,{q + 1}}}},{1 \leq i \leq {q.}}}} & \left. 8 \right) \end{matrix}$

These will be used in equation 3) in the detecting unit 203.

2. Mathematical Support for Equation 7)

We first analyze a column of counters in the counting units 301. For this mathematical analysis, we still use m to denote the number of counters, while in the specification of the system modules, we use w to denote this number. After this, we will analyze combining three columns of counters.

2.1 A Single Column of Counters

After updating the column of counters with all packets, we use c₁, . . . , c_(m) to denote the counter values (while in the specification of the system modules, we use M_(x)[i] to denote the counter values). We use y_(k) to denote the number of counters with value k. In addition, we use n to represent the total flow count, and use n_(k) to represent the number of flows with exactly k packets.

Now consider a counter c[l] whose value is zero, which means no flow maps into position l. This probability is expected to be

$\left( {1 - \frac{1}{m}}\; \right)^{n} \approx {^{- \frac{n}{m}}.}$

While the probability of a counter has a value of one is

${\begin{pmatrix} n_{1} \\ 1 \end{pmatrix}\frac{1}{m}\left( {1 - \frac{1}{m}} \right)^{n_{1} - 1}\left( {1 - \frac{1}{m}} \right)\sum\limits_{i \geq 2^{n_{i}}}} \approx {\frac{n_{1}}{m}{^{{- n}/m}.}}$

Formally, we have

Lemma 1. When n and in simultaneously approach infinity (lim_(m→x)n²/m), we have lim_(m→x)E(y₀)=me^(−a). lim_(m→x)E(y₁)=n₁e^(−a).

where

=lim_(m→x)n/m. Furthermore, y₀ and y₁ are the MLEs (Maximum Likelihood Estimator) of E(y₀) and E(y₁) respectively.

Proof of Lemma 1: We consider the limit distributions of y₀ and y₁ in two propositions.

Proposition 1. When n, n₁ and m simultaneously tend to infinity, the limit distributions of y₀ and y₁ both converge to Poisson, with expectations converging to

$\lambda_{0} = {m\; ^{- \frac{n}{m}}}$

and

$\lambda_{1} = {n_{1}^{- \frac{n}{m}}}$

respectively, if λ₀ and λ₁ remain bounded, as proved as follows:

It has been proved in the literature that, with n balls, m urns, let p_(n,m)(t) be the probability that exactly t urns are empty, then p_(n,m)(t) converges to Poisson distribution with parameter

${\lambda = {m\; ^{- \frac{n}{m}}}},$

if n and m tend to infinity so that

$\lambda = {m\; ^{- \frac{n}{m}}}$

remains bounded. If we have n flows and m counters, the empty counters after seeing all packets will be exactly the same as the empty urns in the urn model, so y₀ also converges to Poisson distribution.

We give a proof sketch for the case of Let q_(n,m)(t) be the probability that there are t counters with value 1 after seeing all packets, then the corresponding packets can only be from the n₁ singleton flows, and

${q_{n,m}(t)} = {\frac{{t!}\begin{pmatrix} n_{1} \\ t \end{pmatrix}\begin{pmatrix} m \\ t \end{pmatrix}\left( {m - t} \right)^{n - t} \times {q_{{n - t},{m - t}}(0)}}{m^{n}}.}$

Take for abbreviation

${{R_{n,m}(v)} = \frac{{v!}\begin{pmatrix} n_{1} \\ v \end{pmatrix}\begin{pmatrix} m \\ v \end{pmatrix} \times \left( {m - v} \right)^{n - v}}{m^{n}}},$

and using the rule of inclusion and exclusion, we can get

${q_{n,m}(0)} = {\sum\limits_{v = 0}^{m}{\left( {- 1} \right)^{v}{{R_{n,m}(v)}.}}}$

Since

${{\left( {m - v} \right)^{v}\left( {1 - \frac{v}{m}} \right)^{n}} < {{v!}\begin{pmatrix} m \\ v \end{pmatrix}\left( {1 - \frac{v}{m}} \right)^{n}} < {m^{v}\left( {1 - \frac{v}{m}} \right)}^{n}},$

and

${\left( \frac{n_{1} - v}{m - v} \right)^{v} < \frac{{v!}\begin{pmatrix} n_{1} \\ v \end{pmatrix}}{\left( {m - v} \right)^{v}} < \left( \frac{n_{1}}{m - v} \right)^{v}},$

multiply these two inequalities, we can get

${\left( \frac{n_{1} - v}{m - v} \right){{{}_{}^{}{}_{}^{}}\left( {1 - \frac{v}{m}} \right)}^{n + v}} < {{v!}{R_{n,m}(v)}} < {\left( \frac{n_{1}}{m - v} \right){{{{}_{}^{v\;}{}_{}^{}}\left( {1 - \frac{v}{m}} \right)}^{n}.}}$

And since

${t < {- {\log \left( {1 - t} \right)}} < \frac{t}{1 - t}},$

we have

${^{- \frac{t}{1 - t}} < {1 - t} < ^{- t}},$

so

$\left( {m\frac{\; {n_{1} - v}}{m - v}^{- \frac{n + v}{m - v}}} \right)^{v} = {{{\left( \frac{n_{1} - v}{m - v} \right){{{}_{}^{}{}_{}^{}}\left( ^{- \frac{v/m}{1 - {v/m}}} \right)}^{n + v}} < {{v!}{R_{n,m}(v)}} < {\left( \frac{n_{1}}{m - v} \right){{{}_{}^{}{}_{}^{}}\left( ^{- \frac{v}{m}} \right)}^{n}}} = {\left( {m\; \frac{n_{1}}{m - v}^{- \frac{n}{m}}} \right)^{v}.}}$

For each fixed v, the extreme ratio of the left side and the right side tends to 1, so we have

${{0 < {\frac{\lambda^{v}}{v!} - {R_{n,m}(v)}}}->0},$

if n and m increase and

$\lambda = {n_{1}^{- \frac{n}{m}}}$

is constrained to a finite interval 0<a<λ<b. If λ tends to 0,

${0 \leq {\frac{\lambda^{v}}{v!} - {R_{n,m}(v)}}}->0$

also holds. So we can get

${\left. {R_{n,m}(v)} \right.\sim\frac{\lambda^{v}}{v!}}.$

Now

${^{- \lambda} - {q_{n,m}(0)}} = {\sum\limits_{v = 0}^{\infty}{\left( {- 1} \right)^{v}\left\{ {\frac{\lambda^{v}}{v!} - {R_{{n,m}\;}(v)}} \right\}}}$

tends to 0, so we have q_(n,m)(0)˜e^(−λ), which also implies q_(n−t,m−t)(0)˜e^(−λ), and

${q_{n,m}(t)} = {{R_{n,m}(t)} \times {\left. {q_{{n - t},{m - t}}(0)} \right.\sim\frac{\lambda^{t}}{t!}}{^{- \lambda}.}}$

Proposition 2. When n, n₁ and m simultaneously tend to infinity, if

$m\; ^{- \frac{n}{m}}\mspace{14mu} {and}\mspace{14mu} n_{1}^{- \frac{n}{m}}$

do not remain bounded, but

$\alpha = {{\lim\limits_{m->\infty}{\frac{n}{m}\mspace{14mu} {and}\mspace{14mu} \alpha_{1}}} = {\lim\limits_{m->\infty}\frac{n_{1}}{m}}}$

are bounded, the limit distributions of y₀ and y₁ converge to normal distribution, with

${{\lim\limits_{m->\infty}{E\left( y_{0} \right)}} = {m\; ^{- \alpha}}},{{\lim\limits_{m->\infty}{{var}\left( y_{0} \right)}} = {m\; {^{- \alpha}\left( {1 - {^{- \alpha}\left( {1 + \alpha} \right)}} \right)}}},{and}$ ${{\lim\limits_{m->\infty}{E\left( y_{1} \right)}} = {n_{1}^{- \alpha}}},{{\lim\limits_{m->\infty}{{var}\left( y_{1} \right)}} = {n_{1}{{^{- \alpha}\left( {1 - {\alpha_{1}{^{- \alpha}\left( {{- 1} + \frac{1}{\alpha_{1}} + \alpha} \right)}}} \right)}.}}}$

The case for y₀ is again the same as that in the urn model, and the case for y₁ can be proved based on the following probability theory which was proved in “H. Geiringer. On the probability theory of arbitrarily linked events. Annals of Mathematical Statistics, 9(4):260-271, 1938.”

Theorem 3. Let p₁, p₂, . . . , p_(m) be the probability of m events E₁, E₂, . . . , E_(m), p_(ij) be the probability of the simultaneous occurrence of E_(i) and E_(j), _(Pijk) that of E_(i), E_(j), E_(k) and finally p_(12 . . . m) that of E₁, E₂, . . . , E_(m). We want to determine the probability P_(m) (x) that x of the m events will take place. We consider the convergence of

${{V_{m}\; (x)} = {\sum\limits_{t \leq x}{P_{m}(t)}}},$

which is the CDF of r. Denoting the mean value of x for V_(m)(x) by a_(m)=Σ_(x−1) ^(m)xP_(m)(x), and denoting the variance of x for V_(m)(x) by s_(m) ², let

${{S_{m}(0)} = 1},{{S_{m}(1)} = {\overset{1\mspace{14mu} \ldots \mspace{14mu} n}{\sum\limits_{i}}p_{i}}},{{S_{n}(2)} = {\overset{1\mspace{14mu} \ldots \mspace{14mu} n}{\sum\limits_{i,j}}p_{ij}}},{{S_{n}(3)} = {\sum\limits_{i,j,k}^{1\mspace{14mu} \ldots \mspace{14mu} n}p_{ijk}}},{{\ldots \mspace{14mu} {S_{m}(m)}} = {p_{12\mspace{14mu} \ldots \mspace{14mu} m}.}}$

If we introduce the following function of the discontinuous variable z=0, 1. 2, . . . , m

${{g_{m}\; (z)} = {\frac{z + 1}{a_{m}}\frac{S_{m}\left( {z + 1} \right)}{S_{m}(z)}}},$

and put z=a_(m)u where u is regarded as a continuous variable in the interval from 0 to ε(ε>0). If h_(m)(u)=g_(m)(z) satisfies the following conditions:

1) If m is sufficiently large, h_(m)(u) admits derivatives of every order in the interval (0, ε).

2) At u=0, the first derivative of h_(m)(u) has a limit, for m→∞, which is different from −1.

3) If u is in the interval (0, ε), the k-th derivatives of h_(m)(u) remains, for every k, inferior to a bound N_(k) which is independent of m.

Then the asymptotic behavior of P_(n)(x) converges towards a normal distribution, that is

${{\lim\limits_{n->\infty}{V_{m}\left( {a_{m} + {{ys}_{m}\sqrt{2}}} \right)}} = {\frac{1}{\sqrt{\pi}}{\int_{- \infty}^{y}{^{- x^{2}}{x}}}}},{{{and}\mspace{14mu} {\lim\limits_{m->\infty}\frac{s_{m}^{2}}{a_{m}}}} = {1 + {{h_{m}^{\prime}(0)}.}}}$

Now following theorem 3, we have

${{S_{m}(z)} = \frac{\begin{pmatrix} m \\ z \end{pmatrix}\begin{pmatrix} n_{1} \\ z \end{pmatrix}{z!} \times \left( {m - z} \right)^{n - z}}{m^{n}}},$

and

$\begin{matrix} {{g_{m}(z)} = {\frac{z + 1}{a_{m}} \times \frac{\begin{pmatrix} m \\ {z + 1} \end{pmatrix}\begin{pmatrix} n_{1} \\ {z + 1} \end{pmatrix}{\left( {z + 1} \right)!}}{\begin{pmatrix} m \\ z \end{pmatrix}\begin{pmatrix} n_{1} \\ z \end{pmatrix}{z!}} \times \frac{\left( {m - \left( {z + 1} \right)} \right)^{n - {({z + 1})}}}{\left( {m - z} \right)^{n - z}}}} \\ {= {\frac{z + 1}{{{mn}_{1}\left( \frac{1}{m} \right)}^{1}\left( {1 - \frac{1}{m}} \right)^{n - 1}} \times \frac{\begin{pmatrix} m \\ {z + 1} \end{pmatrix}\begin{pmatrix} n_{1} \\ {z + 1} \end{pmatrix}{\left( {z + 1} \right)!}}{\begin{pmatrix} m \\ z \end{pmatrix}\begin{pmatrix} n_{1} \\ z \end{pmatrix}{z!}} \times}} \\ {\frac{\left( {m - \left( {z + 1} \right)} \right)^{n - {({z + 1})}}}{\left( {m - z} \right)^{n - z}}} \\ {= {\frac{m - z}{m} \times \frac{n_{1} - z}{n_{1}} \times \frac{m^{n}}{\left( {m - 1} \right)^{n - 1}} \times \frac{\left( {m - \left( {z + 1} \right)} \right)^{n - {({z + 1})}}}{\left( {m - z} \right)^{n - z}}}} \\ {= {\frac{m - z}{m} \times \frac{n_{1} - z}{n_{1}} \times}} \\ {{\left\lbrack {\left( \frac{1 - \frac{z + 1}{m}}{\left( {1 - \frac{z}{m}} \right)\left( {1 - \frac{1}{m}} \right)} \right)^{ma} \times \frac{\left( {1 - \frac{1}{m}} \right)\left( {1 - \frac{z}{m}} \right)^{z}}{\left( {1 - \frac{z + 1}{m}} \right)^{z + 1}}} \right\rbrack.}} \end{matrix}$

Plug

$\alpha_{1} = \frac{n_{1}}{m}$

into it, alto introduce a new variable

${v = {\frac{z}{m} = {u\frac{a_{m}}{m}}}},$

since a_(m) is of the order of magnitude of m, we get

${{\overset{\_}{h}}_{m}(v)} = {{g_{m}(z)} = {\left( {1 - v} \right)\frac{\alpha_{1} - v}{\alpha_{1}} \times {\left\lbrack {\left( \frac{1 - \frac{1}{m\left( {1 - v} \right)}}{1 - \frac{1}{m}} \right)^{ma} \times \frac{1 - \frac{1}{m}}{1 - v - \frac{1}{m}} \times \left( {1 - \frac{1}{m\left( {1 - v} \right)}} \right)^{- {mv}}} \right\rbrack.}}}$

We can easily see that the condition 1) and 3) in theorem 3 are satisfied (if ε<1) since the k-th derivatives of h _(m)(v) contains only rational expressions of (1−v) and positive powers of

${\log \left( {1 - \frac{1}{m\left( {1 - v} \right)}} \right)},$

and

$\begin{matrix} {{\lim\limits_{m\rightarrow\infty}\left( \frac{{{\overset{\_}{h}}_{m}(v)}}{v} \right)_{v = 0}} = {\lim\limits_{m\rightarrow\infty}\left\{ {{- 1} - \frac{1}{\alpha_{1}} + \begin{bmatrix} {{- \frac{\alpha}{1 - \frac{1}{m}}} + \frac{1}{1 - \frac{1}{m}} -} \\ {m\; {\log \left( {1 - \frac{1}{m}} \right)}} \end{bmatrix}} \right\}}} \\ {{= {1 - \frac{1}{\alpha_{1}} - \alpha}},} \end{matrix}$ so $\begin{matrix} {{\lim\limits_{m\rightarrow\infty}\left( \frac{{h_{m}(u)}}{u} \right)_{v = 0}} = {\lim\limits_{m\rightarrow\infty}{\left( \frac{{{\overset{\_}{h}}_{m}(v)}}{v} \right)_{v = 0} \times {\lim\limits_{m\rightarrow\infty}\frac{a_{m}}{m}}}}} \\ {= {\alpha_{1}{{^{- \alpha}\left( {1 - \frac{1}{\alpha_{1}} - \alpha} \right)}.}}} \end{matrix}$

Let

${{g\left( \alpha_{1} \right)} = {\alpha_{1}{^{- \alpha}\left( {1 - \frac{1}{\alpha_{1}} - \alpha} \right)}}},$

we must prove g(a₁)>−1 as stated in condition 2) of theorem 3.

Let f(a)=e^(−a)(1−1/a−a)=e^(−a)(−a²+a−1), then its derivative f(a)=e^(−a)(a−1)(a−2), so the local min/max values are obtained at a=0, 1, 2, since a>0, and f′(1)=f′(2)=0. Since f″(a)=−e^(−a)(a²−5a+5), f″(1)<0 and f″(2)>0, the minimum values may be obtained at a=0 or 2. Since f(0)=−1 and f(2)=−3e⁻², and a>0, we get f(a)>−1.

Now g(a₁)−f(a)=e^(−a)(a−a₁)(a−1) , since a₁≦a, if a≧1, we can get g(a₁)≧f(a)>−1. If a<1, that is 0<a₁≦a₁. Now g(a₁)=e^(−a)(1−a)a₁−e^(−a)>e^(a)>−1.

So, we finally get

${{\lim\limits_{m\rightarrow\infty}{h_{m}^{\prime}(0)}} = {{g\left( \alpha_{1} \right)} > {- 1}}},$

then we can conclude that, y₁ converges to normal distribution with

${\lim\limits_{m\rightarrow\infty}\frac{s_{m}^{2}}{a_{m}}} = {1 - {\alpha_{1}{{^{- \alpha}\left( {{- 1} + \frac{1}{\alpha_{1}} + \alpha} \right)}.}}}$

With simple manipulation, we get the results stated in proposition 2.

With Proposition 1 and 2, we have proved Lemma 1, since the sample mean is the MLE of the expectation of a Poisson or Normal distribution.▪

Lemma 2. The number of singleton flows, n₁, can be estimated asymptotically unbiased by an MLE {circumflex over (n)}₁=m×y₁/y₀. The variance is approximately n₁((e^(a)−2a)a₁+e^(a)−1), where a₁=lim_(m→∝)n₁/m.

Proof for Lemma 2: We prove four propositions step by step.

Proposition 1. The two variables y₀ and y₁ are asymptotically independent.

Let us consider such a reordering of the packet arrivals that all flows of size more than one arrive before any singleton flow. Since such a reorder will not change the distribution of counter values, we will get the same probabilities. Assume right before any singleton flow arrives, exactly k counters remain empty. Now only those singleton flows which map into these k counters are able to induce counter value of zero or one, since the other m-k counters have value at least two. Assume n₁′ singleton flows map into these k counters, then this is just a classical occupancy problem, where exactly y₀ counters have value zero and y₁ counters have value one. The coefficient of y₀ and y₁ in such a classical occupancy problem can almost be neglected when n₁/m is not around 1 (following “Random Allocations”, equation (9), pp. 38), and since under most conditions, y₁ and y₀ are normal and their joint distribution is multivariate normal, they are asymptotically conditional independent for any given k, thus y₁ and y₀ are asymptotically independent.

Proposition 2. Asymptotically, {circumflex over (n)}₁ is a Maximum Likelihood Estimator (MLE) of n₁ From Lemma 1, we know

${{\lim\limits_{m\rightarrow\infty}\frac{E\left( y_{1} \right)}{E\left( y_{0} \right)}} = \frac{n_{1}}{m}},$

that is

$n_{1} = {m \times {\lim\limits_{m\rightarrow\infty}{\frac{E\left( y_{1} \right)}{E\left( y_{0} \right)}.}}}$

Since if {circumflex over (X)}₁, {circumflex over (X)}₂, . . . , {circumflex over (X)}_(t) are the MLEs of variables X₁, X₂, . . . , X_(t) respectively, and if function τ(X₁, X₂, . . . , X_(t)) is a transform of the parameter space of (X₁, X₂, . . . , X_(t)), then τ({circumflex over (X)}₁, {circumflex over (X)}₂, . . . , {circumflex over (X)}_(t)) is the MLE of τ(X₁, X₂, . . . , X_(t)), and by Lemma 1, y₀ and y₁ are the MLEs of E(y₀) and E(y₁) respectively, we get that {circumflex over (n)}₁ is the MLE of n₁.

Proposition 3. The estimator {circumflex over (n)}₁ asymptotically unbiased.

Expanding f(X) by Taylor series, provided that f is thrice differentiable and that the mean and variance of X are finite, it will have

$\begin{matrix} {{E\left\lbrack {f(X)} \right\rbrack} = {E\begin{bmatrix} {{f\left( {E(X)} \right)} + {\left\lbrack {X - {E(X)}} \right\rbrack f^{\prime}\left( {E(X)} \right)} +} \\ {{\frac{\left\lbrack {X - {E(X)}} \right\rbrack^{2}}{2}{f^{''}\left( {E(X)} \right)}} + \ldots} \end{bmatrix}}} \\ {\approx {{E\left\lbrack {f\left( {E(X)} \right)} \right\rbrack} + {E\left\lbrack {\left\lbrack {X - {E(X)}} \right\rbrack {f^{\prime}\left( {E(X)} \right)}} \right\rbrack} +}} \\ {{E\left\lbrack {\frac{\left\lbrack {X - {E(X)}} \right\rbrack^{2}}{2}{f^{''}\left( {E(X)} \right)}} \right\rbrack}} \\ {= {{f\left( {E(X)} \right)} + {\frac{{var}(X)}{2}{{f^{''}\left( {E(X)} \right)}.}}}} \end{matrix}$

So for

${{f\left( y_{0} \right)} = \frac{1}{y_{0}}},$

we have

${E\left( \frac{1}{y_{0}} \right)} \approx {\frac{1}{E\left( y_{0} \right)} + {\frac{{var}\left( y_{0} \right)}{E^{3}\left( y_{0} \right)}.}}$

Since y₀ and y₁ are two independent random variables, we have

$\begin{matrix} {{E\left( {\hat{n}}_{1} \right)} = {m \times {E\left( y_{1} \right)} \times {E\left( \frac{1}{y_{0}} \right)}}} \\ {\approx {m \times {E\left( y_{1} \right)} \times \left( {\frac{1}{E\left( y_{0} \right)} + \frac{{var}\left( y_{0} \right)}{E^{3}\left( y_{0} \right)}} \right)}} \\ {= {m \times n_{1}^{- \alpha} \times \left( {\frac{1}{m\; ^{- \alpha}} + \frac{m\; {^{- \alpha}\left( {1 - {^{- \alpha}\left( {1 + \alpha} \right)}} \right)}}{\left( {m\; ^{- \alpha}} \right)^{3}}} \right)}} \\ {= {n_{1}\left( {1 + \frac{\left( {1 - {^{- \alpha}\left( {1 + \alpha} \right)}} \right)}{m\; ^{- \alpha}}} \right)}} \\ {= {n_{1} + {\frac{n_{1}\left( {1 - {^{- \alpha}\left( {1 + \alpha} \right)}} \right)}{m\; ^{- \alpha}}.}}} \end{matrix}$

Since

${{\lim\limits_{m\rightarrow\infty}{{bias}\left( \frac{{\hat{n}}_{1}}{n_{1}} \right)}} = {{\lim\limits_{m\rightarrow\infty}\frac{^{\alpha} - \left( {1 + \alpha} \right)}{m}} = 0}},$

is asymptotically unbiased.

Proposition 4. Asymptotically,

${{var}\left( {\hat{n}}_{1} \right)} \approx {{\left( {^{\alpha} - {2\alpha}} \right)\frac{n_{1}^{2}}{m}} + {\left( {^{\alpha} - 1} \right){n_{1}.}}}$

Similarly, using Taylor expansion,

var[f(X)] = var[f(E(X)) + [X − E(X)]f^(′)(E(X))) + …  ] ≈ [f^(′)(E(X))]²var(X).     so $\mspace{79mu} {{{{var}\left( \frac{1}{y_{0}} \right)} \approx {\left\lbrack {- \frac{1}{E^{2}\left( y_{0} \right)}} \right\rbrack^{2}{{var}\left( y_{0} \right)}}} = {\frac{{var}\left( y_{0} \right)}{E^{4}\left( y_{0} \right)}.}}$

For independent random variables A and B, it will have

var(AB) = var(A)E²(B) + var(B)E²(A) + var(A)var(B), so  we  have $\begin{matrix} {{{var}\left( \frac{y_{1}}{y_{0}} \right)} = {{{{var}\left( y_{1} \right)}{E^{2}\left( \frac{1}{y_{0}} \right)}} + {{{var}\left( \frac{1}{y_{0}} \right)}{E^{2}\left( y_{1} \right)}} + {{{var}\left( y_{1} \right)}{{var}\left( \frac{1}{y_{0}} \right)}}}} \\ {\approx {{{{var}\left( y_{1} \right)}\left\lbrack {\frac{1}{E\left( y_{0} \right)} + \frac{{var}\left( y_{0} \right)}{E^{3}\left( y_{0} \right)}} \right\rbrack}^{2} +}} \\ {{{\frac{{var}\left( y_{0} \right)}{E^{4}\left( y_{0} \right)}{E^{2}\left( y_{1} \right)}} + {{{var}\left( y_{1} \right)}\frac{{var}\left( y_{0} \right)}{E^{4}\left( y_{0} \right)}}}} \\ {= {{\frac{{var}\left( y_{1} \right)}{E^{2}\left( y_{0} \right)}\left\lbrack {1 + {3\; \frac{{var}\left( y_{0} \right)}{E^{2}\left( y_{0} \right)}} + \left( \frac{{var}\left( y_{0} \right)}{E^{2}\left( y_{0} \right)} \right)^{2}} \right\rbrack} +}} \\ {{\frac{{var}\left( y_{0} \right)}{E^{2}\left( y_{0} \right)}{\frac{E^{2}\left( y_{1} \right)}{E^{2}\left( y_{0} \right)}.}}} \end{matrix}$

Denoting by

${M = {\frac{{var}\left( y_{0} \right)}{E^{2}\left( y_{0} \right)} = {\left. \frac{^{\alpha} - 1 - \alpha}{m} \right.\sim{O\left( \frac{1}{m} \right)}}}},$

we have

$\begin{matrix} \begin{matrix} {{{var}\left( \frac{y_{1}}{y_{0}} \right)} \approx {{\frac{{var}\left( y_{1} \right)}{{var}\left( y_{0} \right)}{M\left( {1 + {3M} + M^{2}} \right)}} + {\frac{E^{2}\left( y_{1} \right)}{E^{2}\left( y_{0} \right)}M}}} \\ {{\approx {\left\lbrack {\frac{{var}\left( y_{1} \right)}{{var}\left( y_{0} \right)} + \frac{E^{2}\left( y_{1} \right)}{E^{2}\left( y_{0} \right)}} \right\rbrack \times M}},} \\ {{{var}\left( {\hat{n}}_{1} \right)} = {m^{2} \times {{var}\left( \frac{y_{1}}{y_{0}} \right)}}} \\ {\approx {m^{2} \times \left\lbrack {\frac{{var}\left( y_{1} \right)}{{var}\left( y_{0} \right)} + \frac{E^{2}\left( y_{1} \right)}{E^{2}\left( y_{0} \right)}} \right\rbrack \times \frac{^{\alpha} - 1 - \alpha}{m}}} \\ {= {{m\left( {^{\alpha} - 1 - \alpha} \right)} \times \left\lbrack {{\alpha_{1}\left( {1 + \frac{\alpha_{1} - {\alpha_{1}\alpha} + \alpha}{^{\alpha} - 1 - \alpha}} \right)} + \alpha_{1}^{2}} \right\rbrack}} \\ {= {m \times \left\lbrack {{\left( {^{\alpha} - {2\alpha}} \right)\alpha_{1}^{2}} + {\left( {^{\alpha} - 1} \right)\alpha_{1}}} \right\rbrack}} \\ {= {{\left( {^{\alpha} - {2\alpha}} \right)\frac{n_{1}^{2}}{m}} + {\left( {^{\alpha} - 1} \right){n_{1}.}}}} \end{matrix} & \; \end{matrix}$

So, if n₁ is very small, the variance is also small, and if n₁ is large, the relative standard error is

${{{stderr}\left( {\hat{n}}_{1} \right)} = {\sqrt{{var}\left( \frac{{\hat{n}}_{1}}{n_{1}} \right)} \approx \sqrt{\frac{^{\alpha} - {2\alpha}}{m} + \frac{^{\alpha} - 1}{n_{1}}}}},$

and this ensures that it is a good estimator for super scanners with a large n₁ if we keep to large enough, and we use it as a cardinality sketch in our estimating module.▪

Lemma 3. For estimating the number of small flows, we can use the following iterative equation to estimate n₁, n₂, . . . , n_(q) step by step:

$\begin{matrix} {{\hat{n}}_{q} = {m \times \left( {\frac{y_{q}}{y_{0}} - {\sum\limits_{i = 1}^{{B_{q}} - 1}\left( {\prod\limits_{d = 1}^{q - 1}\; \frac{\left( {{\hat{n}}_{d}/m} \right)^{\beta_{i{(d)}}}}{\beta_{i{(d)}}!}} \right)}} \right)}} & \left. 9 \right) \end{matrix}$

Proof for Lemma 3: Use

_(q) to denote the set of different flow set patterns that have exactly q packets, and use β_(i) (1≦i≦

_(q)|) to denote a specific flow set pattern in

_(q), so there are totally q packets in β_(i), and a counter with a value of q will correspond to some β_(i). Suppose β_(i) is composed of β_(i(1)) flows of size one, β_(i(2)) flows of size two, . . . , β_(i(q)) flows of size q, and no flow of size more than q, we can write

β_(i)=<β_(i(1)), β_(i(2)), . . . , βhd i(q), 0, . . . >

and order them: β₁=<q, 0, . . . , 0, 0, . . . > corresponding to q flows of size one, β₂=<q−2, 1, 0, . . . , 0, 0, . . . > corresponding to q-2 flows of size one and one flow of size two, and β_(|βq|)=<0, 0, . . . , 1, 0, . . . > (the q-th element is 1) corresponding to one flow of size q. Denoting the number of counters with value x by y_(x), and the number of flows of size x by n_(x), when m is large, we have

${{E\left( \frac{y_{q}}{m} \right)} = {\sum\limits_{i = 1}^{B_{q}}{\Pr \left( \beta_{i} \right)}}},$

where Pr(β_(i)), the probability that the flow pattern β_(i) appears, is

$\prod\limits_{d = 1}^{q}\; {{C_{n_{d}}^{\beta_{i{(d)}}}\left( \frac{1}{m} \right)}^{\beta_{i{(d)}}}\left( {1 - \frac{1}{m}} \right)^{n_{d} - \beta_{i{(d)}}}{\prod\limits_{d > q}^{\;}\; {\left( {1 - \frac{1}{m}} \right)^{n_{d}}.}}}$

Then by simple manipulation, we get

${\frac{E\left( y_{q} \right)}{m} \approx {\frac{E\left( y_{0} \right)}{m}\left\lbrack {{\sum\limits_{i = 1}^{{B_{q}} - 1}\left( {\prod\limits_{d = 1}^{q - 1}\; \frac{\left( {n_{d}/m} \right)^{\beta_{i{(d)}}}}{\beta_{i{(d)}}!}} \right)} + \frac{n_{q}}{m}} \right\rbrack}},$

so

${n_{q} \approx {m \times \left( {\frac{E\left( y_{q} \right)}{E\left( y_{0} \right)} - {\sum\limits_{i = 1}^{{B_{q}} - 1}\left( {\prod\limits_{d = 1}^{q - 1}\; \frac{\left( {n_{d}/m} \right)^{\beta_{i{(d)}}}}{\beta_{i{(d)}}!}} \right)}} \right)}},$

where

β_(q) can be pre-computed.▪2.2 Combining Three Columns of Counters

When considering the estimating unit 305, we use three columns of counters. Each column, by the above mathematical analysis, can be used to estimate the total small flow count of all hosts that are mapped to that column. Now we deal with how to combine them to estimate the small flow count of the host that is mapped to exactly the three columns.

Let us denote the three sketches by M₁, M₂ and M₃, and denote the corresponding flows mapped to them by S₁, S₂ and S₃. We denote by |M_(i)|^(k) the number of flows with k packets estimated from M_(i), which is an estimate on |S_(i)|^(k), the exact number of flows with k packets in S_(i), and we abuse k=0 for total flows. So we can compute an estimation for |S_(i)|^(k) as

=|M_(i)|^(k) using equation 9) above. The idea of combining three columns is that, although more than one host can be mapped to each column, there is only a very small probability that more than one host can be simultaneously mapped to the same three columns. So we can just use |S₁∩S₂∩S₃|^(k) as the number of flows with k packets of the host mapped to the three columns.

Now we discuss operations among three sets (S₁, S₂, S₃) and the corresponding columns (M₁, M₂, M₃). The ancillary notations are: S_(ii)=S_(i)−S_(j)−S_(k), S_(ij)=(S^(i)∩S₁)−S_(l), and S=S_(i)∩S_(j)∩S_(l), where i, j, l is a permutation of 1, 2, 3, as shown in FIG. 12. We also denote by M_(i)+M_(j) (M_(i)+M_(j)+M_(l)) the counter-wise addition, such that (M_(i)+M_(j))[i]=M_(i)[i]+M_(j)[i] ((M_(i)+M_(j)+M_(l)[i]=M_(i)[i]+M_(j)[i]++M_(l)[i]), for 1≦i≦m. We also call such a column of counters as a “sketch”. Note that when two sketches are added up, some packets are counted twice, and when three sketches are added up, some packets are counted twice, and some packets are counted thrice.

Case 1: For k=0, we have

${{M_{i}}^{0} = \hat{{S_{i}}^{0}}},{{{M_{i} + M_{j}}}^{0} = {\hat{S_{i}\bigcup S_{j}}}^{0}},{{{M_{i} + M_{j} + M_{l}}}^{k} = {{S_{i}\bigcup\hat{S_{j}}\bigcup S_{l}}}^{0}},$

so we get

$\begin{matrix} \begin{matrix} {\hat{{S}^{0}} = {\hat{{S_{1}}^{0}} + \hat{{S_{2}}^{0}} + \hat{{S_{3}}^{0}} -}} \\ {{\hat{{{S_{1}\bigcup S_{2}}}^{0}} - \hat{{{S_{2}\bigcup S_{3}}}^{0}} - \hat{{{S_{1}\bigcup S_{3}}}^{0}} +}} \\ {\hat{{{S_{1}\bigcup S_{2}\bigcup S_{3}}}^{0}}} \\ {= {{M_{1}}^{0} + {M_{2}}^{0} + {M_{3}}^{0} -}} \\ {{{{M_{1} + M_{2}}}^{0} + {{M_{2} + M_{3}}}^{0} - {{M_{1} + M_{3}}}^{0} +}} \\ {{{{M_{1} + M_{2} + M_{3}}}^{0}.}} \end{matrix} & \left. 10 \right) \end{matrix}$

Case 2: For flow size of k=6x+1 or k=6x+5, we have

${{M_{i}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{ij}}^{k}} + \hat{{S_{ik}}^{k}} + \hat{{S}^{k}}}},{{{M_{i} + M_{j}}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{jj}}^{k}} + \hat{{S_{il}}^{k}} + \hat{{S_{jl}}^{k}}}},{{{M_{i} + M_{j} + M_{l}}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{jj}}^{k}} + {\hat{{S_{ll}}^{k}}.}}}$

so we get

$\begin{matrix} {\hat{{S}^{k}} = {\left( {{M_{1}}^{k} + {M_{2}}^{k} + {M_{3}}^{k} - {{M_{1} + M_{2}}}^{k} - {{M_{2} + M_{3}}}^{k} - {{M_{1} + M_{3}}}^{k} + {{M_{1} + M_{2} + M_{3}}}^{k}} \right) \times {\frac{1}{3}.}}} & \left. 11 \right) \end{matrix}$

Case 3: For flow size of k=6x+2 or k=6x+4, we have

${{M_{i}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{ij}}^{k}} + \hat{{S_{il}}^{k}} + \hat{{S}^{k}}}},{{{M_{i} + M_{j}}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{jj}}^{k}} + \hat{{S_{il}}^{k}} + \hat{{S_{jl}}^{k}} + \hat{{S_{ij}}^{k/2}} + \hat{{S}^{k/2}}}},{{{M_{i} + M_{j} + M_{l}}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{jj}}^{k}} + \hat{{S_{ll}}^{k}} + \hat{{S_{ij}}^{k/2}} + \hat{{S_{il}}^{k/2}} + \hat{{S_{jl}}^{k/2}}}},$

so we have

$\begin{matrix} {{\hat{{S}^{k}} = {{\left( {{M_{1}}^{k} + {M_{2}}^{k} + {M_{3}}^{k} - {{M_{1} + M_{2}}}^{k} - {{M_{2} + M_{3}}}^{k} - {{M_{1} + M_{3}}}^{k} + {{M_{1} + M_{2} + M_{3}}}^{k}} \right) \times \frac{1}{3}} + \hat{{S}^{k/2}}}},} & \left. 12 \right) \end{matrix}$

where |S|^(k/2) can be induced from earlier computations.

Case 4: For flow size of k=6x+3, we have

${{M_{i}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{ij}}^{k}} + \hat{{S_{il}}^{k}} + \hat{{S}^{k}}}},{{{M_{i} + M_{j}}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{jj}}^{k}} + \hat{{S_{il}}^{k}} + \hat{{S_{jl}}^{k}}}},{{{M_{i} + M_{j} + M_{l}}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{jj}}^{k}} + \hat{{S_{ll}}^{k}} + \hat{{S}^{k/3}}}},$

so we get

$\begin{matrix} {{\hat{{S}^{k}} = {{{- \left( {{M_{1}}^{k} + {M_{2}}^{k} + {M_{3}}^{k} - {{M_{1} + M_{2}}}^{k} - {{M_{2} + M_{3}}}^{k} - {{M_{1} + M_{3}}}^{k} + {{M_{1} + M_{2} + M_{3}}}^{k}} \right)} \times \frac{1}{3}} + {\hat{{S}^{k/3}} \times \frac{1}{3}}}},} & \left. 13 \right) \end{matrix}$

where |S|^(k/3) can be induced from earlier computations.

Case 5: For flow size of k=6x (x>0), we have

$\begin{matrix} {{\hat{{S}^{k}} = {{\left( {{M_{1}}^{k} + {M_{2}}^{k} + {M_{3}}^{k} - {{M_{1} + M_{2}}}^{k} - {{M_{2} + M_{3}}}^{k} - {{M_{1} + M_{3}}}^{k} + {{M_{1} + M_{2} + M_{3}}}^{k}} \right) \times \frac{1}{3}} + \hat{{S}^{k/2}} - {\hat{{S}^{k/3}} \times \frac{1}{3}}}},} & \left. 14 \right) \end{matrix}$

so we get

$\mspace{79mu} {{{M_{i}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{ij}}^{k}} + \hat{{S_{il}}^{k}} + \hat{{S}^{k}}}},\mspace{79mu} {{{M_{i} + M_{j}}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{jj}}^{k}} + \hat{{S_{il}}^{k}} + \hat{{S_{jl}}^{k}} + \hat{{S_{ij}}^{k/2}} + \hat{{S}^{k/2}}}},{{{M_{i} + M_{j} + M_{l}}}^{k} = {\hat{{S_{ii}}^{k}} + \hat{{S_{jj}}^{k}} + \hat{{S_{ll}}^{k}} + \hat{{S}^{k/3}} + \hat{{S_{il}}^{k/2}} + \hat{{S_{jl}}^{k/2}} + \hat{{S_{ij}}^{k/2}}}},}$

where |S|^(k/2) and |S|^(k/3) can be induced from earlier computations.

Finally, when estimating the number of flows with no more than q packets, we need to add

together from k=1 to q, that is

St=Σ_(k=1) ^(q)

  15)

A special case is when k=1, so using equation 11), we get

St=(|M ₁|¹ +|M ₂|¹ +|M ₃|¹ −|M ₁ +M ₂|¹ −|M ₂ +M ₃|¹ −|M ₁ +M ₃|¹ +|M ₁ +M ₂ +M ₃|¹)/3.

Combining this with Lemma 2 for estimating singleton flows from a single column, we get equation 7) used in step 905.

The present invention has been illustrated by the above descriptions and embodiments, but the present invention is not limited thereto. Various modifications and changes can be made by those skilled in the art according to the disclosure herein, which should be within the scope of the present invention. 

1. A system for catching top hosts from a plurality of hosts, comprising: a filter (10) configured to sample flows from the hosts and remove the flows that do not satisfy a constraint; a tracker (20) configured to record a first estimated flow count for each host and to determine a first set of hosts from the plurality of hosts in term of the estimated flow count; and an estimator (30) configured to determine a second estimated flow count for each of the determined hosts and select a second set of hosts from the determined hosts based on the second estimated flow count.
 2. A system of claim 1, wherein the filter (10) further comprises: a sampling unit (101) configured to sample the arrival packets of the flows and map each sampled packet into a value associated with a flow ID of the arrival packets; an accounting unit (103) configured with a plurality of counters for recording the mapped value; and a determining unit (102) configured to determine whether the flows satisfy said constraint based on the mapped value, if no, the flows are removed.
 3. A system of claim 2, wherein said constraint is set such that the determining unit (102) removes the flows associated with the value when the value is lager than a predetermined sampling threshold, and the determining unit (102) triggers the accounting unit (103) to update the counters therein when the value is not lager than the predetermined sampling threshold.
 4. A system of claim 2, wherein the accounting unit (103) operates to update the counters therein by rule of: if the value of the counter associated with the mapped value in the plurality of counters is no greater than q, the said counter will increase by one, where q represents a user specified parameter for characterizing small flows of no more than q packets.
 5. A system of claim 2, wherein the data structure in the tracker comprises: a hash table (201) having a plurality of nodes; and a min-heap (202), wherein a key of each node in the hash table (201) is host ID, and a value of each node in the hash table (201) refers to a position of a node in the min-heap (202).
 6. A system of claim 5, wherein each node in the min-heap (202) comprises a field of host ID, a field of count, and a field of error of count, and wherein the tracker (20) further comprises a detecting unit (203) configured to detect the change of counters in the accounting unit (103) so as to update the field of count and error in the min-heap (202).
 7. A system of claim 6 wherein the detecting unit (203) is further configured to update the field of count in the min-heap (202) by rule of If c_(t)[h_(m)(f)]=i and i≦q, count_(s)=count_(s)+U_(i,q)(t) wherein U_(i,q)(t) is set forth by rule of ${Y_{{q - 1},q} = \frac{m}{y_{0}}},{Y_{{q - 2},q} = {{- 1} \times \left\lbrack {Y_{{q - 1},q} \times \frac{y_{1}}{y_{0}}} \right\rbrack}},{Y_{{q - 3},q} = {{- 1} \times \left\lbrack {{Y_{{q - 2},q} \times \frac{y_{1}}{y_{0}}} + {Y_{{q - 1},q} \times \frac{y_{2}}{y_{0}}}} \right\rbrack}},{Y_{{q - i},q} = {{- 1} \times \left\lbrack {\sum\limits_{j = 1}^{i - 1}{Y_{{q - {({i - j})}},q} \times \frac{y_{j}}{y_{0}}}} \right\rbrack}},\mspace{56mu} \ldots$ ${Y_{0,q} = {{{- 1} \times {\left\lbrack {\sum\limits_{j = 1}^{q - 1}{Y_{j,q} \times \frac{y_{j}}{y_{0}}}} \right\rbrack.U_{0,q}}} = {{m/y_{0}} - Y_{0,{q + 1}}}}},{U_{i,q} = {- Y_{i,{q + 1}}}},{1 \leq i \leq {q.}}$ where y_(i) represents number of counters in the accounting unit (103) with value i, q represents a user specified parameter for characterizing small flows of no more than q packets. m represents the number of counters in the accounting unit (103), c_(t)[h_(m)(f)] is the value of the counter associated with the flow with ID f in the accounting unit (103), count_(s) represents the value of the counter associated with the host with ID s in the min-heap (202).
 8. A system of claim 1, wherein the tracker (20) further comprises: a selection unit (204) configured to determine the first set of hosts from the plurality of hosts in term of the first estimated flow counts, wherein the estimated flow counts are determined based on a difference between a count and a count error for each host based on the received packets.
 9. A system of claim 1, wherein the estimator (30) further comprises: a plurality of counting units (301) configured to record information of flow count for the first set of hosts; and an updating unit (304) configured to update the information on the plurality of counting units.
 10. A system of claim 9, wherein the estimator (30) further comprises: a selecting unit (303) configured to select three counting units from a plurality of counting units, and wherein the updating unit (304) is further configured to update the selected counting units by For i=1 to 3 If M _(h) _(i) _((s)) [h _(w)(f)]<=q, M _(h) _(i) _((s)) [h _(w)(f)]=M_(h) _(i) _((s)) [h _(w)(f)]+1 where M_(h) _(i) _((s))[h_(w)(f)] is a selected counting unit from the plurality of counting units (301), and q represents a user specified parameter for characterizing small flows of no more than q packets.
 11. A system of claim 10, wherein the estimator (30) further comprises: an estimating unit (305) configured to estimate flow counts from the determined first set of hosts based on the numbers in the selected counting units.
 12. A method for catching top hosts from a plurality of hosts, comprising: sampling a plurality of packets from the hosts during a determined interval of time; determining a difference between a count and a count error for each of the hosts based on the sampled packets; ranking the hosts based on the determined difference to identify a first set of hosts in the ranked hosts; estimating a flow count for each of the first set of hosts; and selecting a second set of hosts from the first set of hosts as the top hosts.
 13. A method of claim 12, wherein the estimating further comprises: calculating a plurality of sums from a plurality of counting units; determining whether any of sums overflow; if yes, estimating the flow count based on the determined difference and a rate of sampling r, otherwise, estimating the flow count based on counts in the plurality of counting units, the calculated sums, the rate of sampling r, and a threshold for filtering non-top hosts T₂.
 14. A system for catching top hosts, comprising: a flow filter configured to test whether a packet in a flow from a plurality of hosts satisfies a predetermined constraint; an tracker configured to get a plurality of candidate hosts with estimated flow counts thereof based on the test; an estimator configured to select the top hosts from the candidate hosts and output the selected hosts.
 15. The system of claim 14, wherein the flow filter further comprises a sampling module configured to sample flows to be processed.
 16. The system of claim 14, wherein the estimator is further configured to output the selected hosts ranked by the estimated flow counts thereof.
 17. A method for catching top hosts, comprising: testing whether a packet in a flow from one of a plurality of hosts satisfies a predetermined constraint; getting candidate hosts with estimated flow counts thereof based on the testing; selecting a plurality of top hosts from the candidate hosts and update the estimated flow counts thereof; and outputting the selected top hosts.
 18. The method of claim 17, further comprising prior to the testing: sampling the flow; and removing the flow if it does not satisfy the predetermined constraint.
 19. The method of claim 17, wherein the outputting comprises: outputting the selected hosts ranked by the estimated flow counts thereof
 20. A system for catching top hosts, comprising: means for testing whether a packet in a flow from a plurality of hosts satisfies a predetermined constraint; means for getting a plurality of candidate hosts with estimated flow counts thereof based on the test; and means for selecting the top hosts from the candidate hosts and outputting the selected hosts.
 21. The system of claim 20, wherein the means for testing further comprises: means for sampling the flow.
 22. The system of claim 20, wherein the means for selecting and outputting is configured to output the selected hosts ranked by the updated flow counts thereof. 